The invention relates to a hydrocarbon conversion process. More specifically the invention relates to a flow scheme for a hydrocracking process as used in petroleum refineries to convert heavy feed stocks into lighter, higher value distillate streams such as naphtha and jet fuel.
Large quantities of petroleum derived feeds are converted into higher value hydrocarbon fractions by a process referred to as hydrocracking. In this process the heavy feed is contacted with a fixed bed of a solid catalyst in the presence of hydrogen at conditions of high temperature and pressure. This results in a substantial portion of the feed stream molecules being broken down into molecules of smaller size and greater volatility.
Large quantities of petroleum derived hydrocarbons are converted into higher value hydrocarbon fractions used as motor fuels through use of a hydrocracking process unit. In the hydrocracking process the heavy feed is contacted with a fixed bed of a solid catalyst in the presence of hydrogen at conditions of high temperature and pressure which results in a substantial portion of the molecules of the feed stream being broken down into molecules of smaller size and greater volatility. The high economic value of hydrocarbon fuels has led to extensive development of both hydrocracking catalysts and the process technology.
Many molecules in the raw petroleum fraction fed to the hydrocracking process contain significant amounts of organic sulfur and nitrogen. The sulfur and nitrogen must be removed to meet modern fuel specifications. Removal or reduction of the sulfur and nitrogen is also beneficial to the operation of a hydrocracking reactor. The sulfur and nitrogen is removed by a process referred to as hydrotreating. Due to the similarity of the process conditions employed in hydrotreating and hydrocracking the two processes are often integrated into a single overall process unit having separate sequential reactors dedicated to the two reactions and a common product recovery section.
Hydrocracking processes are used commercially in a large number of petroleum refineries. They are used to process a variety of feeds ranging from naphtha to very heavy crude oil residual fractions. In general, the hydrocracking process splits the molecules of the feed into smaller molecules having higher average volatility and economic value. At the same time a hydrocracking process normally improves the quality of the material being processed by increasing the hydrogen to carbon ratio of the materials, and by removing sulfur and nitrogen. The significant economic utility of the hydrocracking process has resulted in a large developmental effort being devoted to the improvement of the process and to the development of better catalysts for use in the process. A general review and classification of the different hydrocracking process flow schemes is provided in the book entitled, xe2x80x9cHydrocracking Science and Technologyxe2x80x9d authored by Julius Scherzer and A. J. Gruia, published in 1996 by Marcel Dekker, Inc. Specific reference is made to the chapter beginning at page 174 which describes single stage, once-through and two-stage hydrocracking process flow schemes and product recovery flows.
U.S. Pat. No. 5,904,835 issued to V. P. Thakkar describes a hydrocracking process in which the feed stream is divided into two portions which are fed into separate hydrocracking reaction zones.
FIG. 2 of U.S. Pat. No. 2,671,754 issued to A. J. DeRosset et al shows a hydrocarbon conversion process having counter-current flow of rising hydrogen and descending hydrocarbons through sequential desulfurization and hydrogenation zones. A similar countercurrent desulfurization and hydrogenation flow is shown in U.S. Pat. No. 3,788,976 issued to M. C. Kirk.
U.S. Pat. No. 4,194,964 issued to N. Y. Chen et al describes a hydrotreater/hydrocracker that can be operated to function as a distillation column. Hydrogen is charged to the bottom of the column and rises countercurrent to liquid phase hydrocarbons fed to the middle of the column.
It has been recognized in the art that the concentration of ammonia in the reaction zone plays an important role in moderating the activity and selectivity of hydrocracking catalysts. This is discussed at page 207 of the Scherzer text. Thus the prior art includes the addition of ammonia to downstream portions of a reaction zone as shown in U.S. Pat. No. 3,859,203 issued to L. W. Brunn et al.
Another processing technique known in the art is the rejuvenation of hydroprocessing catalyst activity by contact with hot flowing hydrogen which strips carbonaceous deposits from the catalyst. This is described in the article appearing at page 165 of the Jun. 6, 1977 edition of the Oil and Gas Journal. A variation of this involving flushing the catalyst with an inert gas is described U.S. Pat. No. 5,817,589 issued to M. Ramirez de Agudelo et al.
The invention is a continuous hydrocracking process characterized by the retention of both hydrotreating and hydrocracking catalysts in each of several parallel countercurrent vapor-liquid flow reaction zones, by several processing steps including internal recycling of unconverted liquid to the inlet of each reaction zone and the combination of vapor removed overhead from each reaction zone to recover product hydrocarbons. Preferably, continuous and substantially uniform long term operation is provided by regenerating the catalysts in one or more reaction zones while the other reaction zones are on stream, with the effluents of all of the reactors being combined to form the stream sent to the product recovery section. The ability to perform frequent regenerations provides a new flexibility in operating conditions. Conditions which would otherwise result in commercially unacceptable short catalyst life (time between required regenerations) can be employed to increase selectivity or reduce operating costs.
One broad embodiment of the invention may be characterized as a hydrocarbon conversion process which comprises dividing a feed stream into a number of portions having the same composition and passing each portion into an upper portion of a separate reaction zone of a multi-reactor reaction section of the process, with the reaction section comprising at least two reaction zones of substantially equal configuration and operated at substantially the same conversion conditions, and with each reaction zone containing an upper first catalyst bed comprising hydrotreating catalyst and a lower second catalyst bed comprising hydrocracking catalyst; passing a hydrogen-rich gas stream into a lower portion of each reaction zone and upward through the reaction zone; collecting liquid phase hydrocarbons at the bottom of each reaction zone and recycling at least a portion of the hydrocarbons to the first catalyst bed of the same reaction zone; and combining vapor rising out of each operating reaction zone and passing the resultant combined gas stream to a product recovery zone.
The process of the invention may also be characterized as a hydrocarbon conversion process which comprises dividing a feed stream into a number of portions having the same composition and passing each portion into an upper portion of a separate on-stream reaction zone of a multi-reaction zone reaction section of the process, with the reaction section comprising at least two reaction zones of substantially equal configuration and operated at substantially the same conversion conditions, and with each reaction zone containing a catalyst bed comprising hydrocracking catalyst; passing a hydrogen-rich gas stream into a lower portion of each reaction zone and upward through the reaction zone; passing liquid phase hydrocarbons collected at the bottom of each on-stream reaction zone into a single liquid retention zone, and recycling a portion of these hydrocarbons to a catalyst bed of each on-stream reaction zone; and, combining vapor rising out of each onstream reaction zone and passing the resultant combined gas stream to a product recovery section of the process, and recovering at least one distillate product from the process.